refactor(nix): remove or integrate unused variable.nix

[siraken]

Summary

nix/modules/variable.nix defines ~10 environment variables but is effectively unused — the import in home.nix is commented out (# sessionVariables = import ...).

Proposed approach

1. Verify no other file references variable.nix
2. If truly unused, delete the file
3. If some variables are still needed, inline them into the relevant program modules (e.g. shell config) instead of maintaining a standalone file